1. Field of the Invention
The present invention relates to liquid crystal devices and particularly concerns improved conversion of unpolarized light to light of a single polarization state.
2. Description of Related Art
Recent developments and wider use of liquid crystal display systems, such as a video projectors, have emphasized the need for increased efficiency in illumination of the liquid crystal devices. The CRT addressed reflective liquid crystal light valve (LCLV) is a thin film, multi-layer structure comprising a liquid crystal layer, a dielectric mirror, a light blocking layer and a photosensitive layer, all sandwiched between two transparent electrodes. In a CRT addressed reflective liquid crystal light valve projection system a polarized illumination (reading) beam is obtained by directing an unpolarized beam from a high intensity light source, such as a xenon arc lamp for example, to a polarizing beam splitter, which divides the incoming light into two beams, one having a P polarization state and the other having an S polarization state. Light of one of these polarization states is passed through the liquid crystal layer to the dielectric mirror, which reflects it back through the liquid crystal layer. The liquid crystal material is optically addressed by an input image of relatively low intensity writing light, such as that generated by a cathode ray tube, the output of which is applied to the photosensitive layer. Therefore, if a complex distribution of light, for example a high resolution input image from the cathode ray tube, is focused on the photosensitive layer, the device converts the relatively low intensity input image into a high intensity replica image which can be reflected for projection with magnification to produce a high brightness image on a large viewing screen.
In active-matrix liquid crystal display (AMLCD) projection systems each pixel is addressed by means of a high intensity light beam of a single polarization state that is directed through a matrix of thin film transistors to provide a projection system that may realize a full color, high quality screen display of large size. The transistors are collectively and selectively activated to provide an image defined by the spatial array of selectively activated transistors.
As compared with conventional cathode ray tube projectors, the liquid crystal light valve projectors have many advantages in regard to compact size, light weight, low cost, and accurate color registration, among others. However, they still exhibit disadvantages in regard to total light output, which is primarily screen brightness, because of the low transmittance of the required polarizing beam splitter and the thin film transistor mode of addressing the active-matrix liquid crystal light valve. These systems can use light of only one polarization state. The polarizing beam splitter, which effectively passes only one half of the light, is necessary for those devices employing twisted nematic liquid crystal mode operation, which can operate on light of only a single polarization state. Accordingly, recovery of otherwise discarded light is essential for efficient operation.
Use of a light source of higher intensity, to overcome high losses, is undesirable. Increased intensity is limited by allowable temperature increase of the projector system components, by available light source power, and by efficiency and economics. A high power light source may cause deterioration of the polarization beam splitter and the liquid crystal light valve itself.
Ability to use all of the light from the high power illumination source has been recognized as highly desirable and will enable a projector of this kind to reduce power consumption by nearly fifty percent, or, for small units, to enable a significant increase of screen brightness.
Recognizing the need to improve efficiency of illumination, systems have been devised for recovery of the otherwise wasted light of the wrong polarization. The polarizing beam splitter passes light of P state polarization and reflects light of S state polarization. Only light of one or the other polarization state, but not both, can be used to illuminate the liquid crystal light valve face. Accordingly, it has been suggested to change the polarization state of the otherwise unused light into light of the useful polarization state and then combine the two so as to theoretically avoid the fifty percent loss of the illumination light. Prior schemes for recovery of the light of nonusable polarization state have included various systems using polarization rotating mirrors and half-wave retarder plates. All have problems. Reflective devices, although changing the direction of linear polarization, introduce a significant amount of depolarization, providing an output having a portion of elliptically polarized or depolarized light. Elliptically polarized light degrades efficiency of operation of the liquid crystal light valve. Further, light reflected from a pair of mirrors and passed through a half-wave retarder for change of polarization is not precisely nor adequately collimated. When combined with the incident light of the useful polarization state from the polarizing beam splitter, further inefficiencies in operation of the liquid crystal light valve are introduced by such lack of collimation.
Importantly, the half-wave retarder plate that has been used in prior polarized light recovery arrangements suffers from chromatic variation of light output. Intensity of the output light of desired polarization from the half-wave retarder is significantly decreased at higher and lower ends of the visible spectrum, so that intensity of both blue and red components of the desired polarization for a full spectrum light beam are decreased with respect to intensity of the intermediate wavelength green component of the desired polarization. This variation across the spectrum can be minimized by decreasing green intensity, but only at the cost of decreased efficiency.
Still further, such prior systems for converting polarization state have combined the original and recovered beam by effectively converging the two so as to be mutually superposed on the illuminated face of the liquid crystal light valve. Such prior devices provide a converging cone angle between the two light sources that is great enough to cause a significant amount of the desired light to fall outside of the desired collection cone angle of the projection lens, and thus to be wasted. In addition, light recovery devices of the prior art are generally complex, expensive and require relatively large numbers of components, thereby requiring a large package and increased costs of manufacture.
Accordingly, it is an object of the present invention to provide a polarized light recovery system that avoids or minimizes above mentioned problems.